1. Field of the Invention
The present invention relates to optical isolators, and particularly to compact isolators having low insertion loss and low vulnerability to external environmental conditions.
2. Description of Prior Art
In present-day optical communications technology, optical signals are typically passed through a plurality of optical interfaces. All interfaces produce reflected signals. Reflected signals which return to a light source through a primary optical route cause the light source to become unstable and noisy. Optical isolators are used to block these reflected signals from reaching the light source. Ideally, optical isolators permit all light rays to move in a forward direction only, and block light rays from moving in a reverse direction.
As shown in FIG. 1, a conventional isolator 1 includes first and second optical collimators 10, an isolated center core 30 and an outer tube 40 enveloping the collimators 10 and the isolated center core 30. Each collimator 10 comprises a stainless steel tube 11, a sleeve 12, a xc2xc pitch Graded Index (GRIN) lens 13, and a ferrule 14 which accommodates an optical fiber 15. The collimators 10 convert input optical signals into parallel rays, for providing sound coupling between two optical devices. The isolated center core 30 is stationed between the two collimators 10 and comprises a first polarizer 31, a Faraday rotator crystal 32, a second polarizer 33, and a toroidal magnetic core 34. The magnetic core 34 envelops the two polarizers 31, 33 and the Faraday rotator crystal 32 to protect them, and provides a magnetic field for the Faraday rotator 32.
Light signals pass in a forward direction from an end of the right-hand input fiber 15 to the right-hand Graded Index (GRIN) lens 13. The GRIN lens 13 collimates the light, and the collimated light from the GRIN lens 13 is then transmitted through the first polarizer 31. The first polarizer 31 is a birefringent crystal wedge. The first polarizer 31 separates incident light from the GRIN lens 13 into an ordinary ray polarized perpendicularly to an optical axis of the first polarizer 31, and an extraordinary ray polarized along the optical axis of the first polarizer 31.
Separation occurs because the birefringent crystal wedge has two indexes of refraction, one for the light polarized along the optical axis and another for the light polarized perpendicularly to the optical axis. The polarized light from the first polarizer 31 is then rotated 45xc2x0 by the Faraday rotator 32. The Faraday rotator 32 is typically formed from garnet doped with impurities, or alternatively YIG, and is placed in the magnetic core 34.
The rotated light rays then enter the second polarizer 33, sometimes called an analyzer. Like the first polarizer 31, the second polarizer 33 typically is a birefringent crystal wedge. An optical axis of the birefringent crystal of the second polarizer 33 is oriented by 45xc2x0 with respect to the optical axis of the birefringent crystal of the first polarizer 31. Thus the ordinary ray from the first polarizer 31 is also an ordinary ray of the second polarizer 33, and the extraordinary ray from the first polarizer 31 is also an extraordinary ray of the second polarizer 33. The result is that after having traveled from the first polarizer 31 through the second polarizer 33, the two polarized rays are recombined by the second polarizer 33. The two polarized rays are then refocused by the left-hand GRIN lens 13 to a point on an end of the left-hand fiber 15.
In the reverse direction, light from the left-hand fiber 15 is separated by the second polarizer 33 into two rays: an ordinary ray polarized perpendicularly to the optical axis of the second polarizer 33, and an extraordinary ray polarized along the optical axis of the second polarizer 33. When passing through the Faraday rotator 32, the light of both rays is rotated 45xc2x0. This rotation is nonreciprocal with the rotation of light in the forward direction. The ordinary ray from the second polarizer 33 is polarized along the optical axis of the first polarizer 31, and the extraordinary ray from the second polarizer 33 is polarized perpendicularly to the optical axis of the first polarizer 31. The ordinary and extraordinary rays from the second polarizer 33 have swapped places incident upon the first polarizer 31. Thus the light, having passed through the first polarizer 31, does not leave the polarizer 31 in parallel rays. The non-parallel light is focused by the right-hand GRIN lens 13 to a point not at the end of the input fiber 15. Thus, light traveling in the reverse direction is not passed back into the right-hand fiber 15.
The conventional optical isolator 1 has its isolated core 30 between the two collimators 10. The left-hand sleeve 12 is within the stainless steel tube 11. The left-hand GRIN lens 13 has a protruding end 131 protruding out of the sleeve 12 into the magnetic core 34. In assembly, the first polarizer 31, the rotator crystal 32 and the second polarizer 33 of the isolated core 30 are stationed within the magnetic core 34. End portions of the polarizers 31, 33 and the rotator crystal 32 are glued to an inner surface of the magnetic core 34. Then the protruding end 131 of the left-hand GRIN lens 13 is glued to the inner surface of the magnetic core 34. The isolated core 30 is thus securely connected with the left-hand collimator 10.
When the magnetic core 34 of the isolated core 30 is glued to the protruding end 131 of the left-hand GRIN lens 13, excess glue may contaminate the GRIN lens 13 and the adjacent surface of the adjacent second polarizer 33. Such contamination reduces the performance of the GRIN lens 13, and results in a large insertion loss of the isolator 1. In addition, such contamination on gelatine surfaces of the GRIN lens 13 and the second polarizer 33 is difficult to remove. Furthermore, the left-hand collimator 10 is fixedly connected with the isolated core 30. It is difficult to adjust the relative position of the collimator 10 and the isolated core 30, so as to accurately focus output light on the end of the left-hand fiber 15. Moreover, the components of the isolator 1 are unduly large. This adds to costs, particularly the cost of the isolated core 30. Finally, the isolated core 30 is located outside the two sleeves 12. Thus the isolated core 30 is vulnerable to changes in temperature of the external environment, which may adversely affect the operation of the isolator 1.
Accordingly, an improved isolator is needed to overcome the many disadvantages of conventional isolators.
Accordingly, one object of the present invention is to provide an optical isolator with low insertion loss and low cost.
Another object of the present invention is to provide an optical isolator which is less vulnerable to changes in its surrounding environment.
A further object of the present invention is to provide an optical isolator which has no contamination caused by excess glue, and which has an isolated core which is effectively insulated from external contaminants both during and after assembly.
To solve the problems of the prior art and achieve the objects set out above, an optical isolator of the present invention comprises a first optical collimator, an isolated core, a second optical collimator and an outer tube. The second collimator has a long sleeve which entirely accommodates the isolated core. The isolated core comprises a first polarizer, a Faraday rotator crystal and a second polarizer stationed in sequence within a toroidal magnetic core. An axial length of the toroidal magnetic core is equal to or slightly less than an overall length of the two polarizers and the rotator crystal. The two polarizers and the rotator crystal are sized such that an overall diameter of the isolated core is less than an inner diameter of the sleeve of the second collimator.
In assembly, a GRIN lens, a ferrule and an optical fiber are fixedly received in the sleeve. The isolated core comprising the toroidal magnetic core, the polarizers and the rotator crystal is glued to an inside of an end of the sleeve. Before the glue cures, the position of the isolated core is adjusted so that the relative positions of the isolated core and the GRIN lens yield optimized optical characteristics. Then, the first collimator and the second collimator incorporating the isolated core are mounted in the outer tube. The relative positions of the first and second collimators are adjusted to obtain optimized optical characteristics for the isolator. Finally, encapsulation is applied to opposite ends of the outer tube to fix the collimators to the outer tube and thereby complete assembly of the isolator.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.